Reversing Persia and the Byzantine Empire

Ok, so this is a bit of an AH challenge. How do we get a reversed situation between the Persian and Byzantine empires in the wake of the Muslim expansions?

Basically, how do we get the Byzantines to fall (there can still be successor states, just no centralized empire) and the Sassanids to lose a lot of territory but still survive (for longer anyways)?
 

Philip

Donor
Basically, how do we get the Byzantines to fall (there can still be successor states, just no centralized empire)

This is rather difficult. Constantinople is nigh impregnable and rather remote from the Arab's base of power. The Romans can trade quite a bit a land while falling back and regrouping. I don't think the Arabs can really press on to Constantinople without securing Persia first.

and the Sassanids to lose a lot of territory but still survive (for longer anyways)?

I could see the Sassanids loosing Mesopotamia while retreating behind the Zagros. But here is the problem: The Arabs can't leave them there and hope to push threaten Constantinople. As soon as the Arabs turn north, the Sassanids can cross the Zagros and retake Mesopotamia.
 
Perhaps if Constantinople somehow falls to the Sassanids and Avars in 626, and is sacked / burned to the ground. If the Byzantine Empire even survives, it would be weak enough to fall to the Arabs, maybe while a stronger Sassanid Persia is able to survive?
 

Philip

Donor
Yeah, I was thinking something involving the Avars and/or Slavs could be an option...

That's another story. The Bulgarians may have had a chance, but I have a hard time seeing them taking Constantinople. On the other hand, it shouldn't be too hard to construct a scenario that has a Bulgarian Master of Soldiers declaring himself (or a lackey) emperor from inside Constantinople. A civil war could be expected. The Bulgarians could end up with the European half of the empire.

Of course, this would probably at least 100-150 years latter than when the Sassanids fell OTL
 
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